It is known that one of the essential problems encountered in this area concerns the proper tolerance of the hearth electrode over time.
This, in fact, is exposed to very severe thermal conditions due at once to the presence of molten metal in the furnace, whose temperature can reach, even exceed, 1800 degrees C., and an internal heating by the joule effect due to very high intensities of the electrical current which passes through it and which, for example, in arc melting furnaces, are about 30,000 amperes.
A solution, recently presented by the applicant in French Patent Application no. 84.10482 of June 27, 1984 consists of making a hearth electrode for an arc furnace from a steel bar going out of the furnace by an end portion covered with a copper sleeve, energy-cooled by a circulation of water and connected to an electricity supply terminal. The application of the sleeve to the circumference of the bar ensures the desired thermal and electrical contacts. As a variation, a nipple made of heat-resistant material extends the external portion of the bar and works with the end of the sleeve to eliminate any risk of the escape of molten metal in case, following possible trouble in cooling, the cone-shaped melting of the central zone of the bar proceeds up to its end.
This electrode gives full satisfaction in periods of operation of the furnace from medium and long durations. Nonetheless, over very long periods of time, it can be feared that, after repetitive sequences of remelting and resolidification of the bar, the effects of differential expansion between the cooled sleeve and the alternately hot and cold bar progressively lead to a deterioration of the quality of the electrical contact between the bar and the sleeve, and even the thermal exchange between these.